Powdery mildew is a group of fungal diseases that affect a variety of different plants. It appears at first as small spots of white on leaf surfaces. These spots can expand until the entire leaf is covered, giving it a powdery appearance.  Sometimes leaves are twisted and contorted. Eventually, leaves can turn yellow and drop off prematurely. The disease can also appear on stems and flower petals, and there are some plants that can be infected without the characteristic powdery appearance.

This is often the time of year when powdery mildew becomes noticeable.  Humid conditions favor infection and dry conditions favor the transfer of spores by wind. This time of year is often hot and dry, and drought-stressed plants are more susceptible.

Although it can be disfiguring, the good news is, powdery mildew is not usually considered fatal for perennials or woody plants. But, the yield of vegetable plants can be significantly impacted by powdery mildew. And significant, repeated infections of woody plants can impact plant health and affect winter hardiness.

Several strategies can be used to prevent powdery mildew. Maintain plant health by placing plants in the right conditions and meeting cultural needs, including adequate moisture. Space plants properly to allow adequate light and air circulation.  Avoid over-fertilizing, as over-stimulated succulent growth is more susceptible to infection. For plants prone to powdery mildew, plant resistant cultivars.

If you notice a small number of infected leaves, you can remove and dispose of them. (Typical garden composting temperatures will not destroy the disease spores). Try to improve air circulation around the plant and prevent drought stress. Small amounts of powdery mildew that are not impacting plant performance can usually just be tolerated, especially late in the season when plants will be losing leaves or top growth soon anyway. Infested leaves or top growth should be removed in fall to prevent disease spores from wintering over.

Fungicides can be used to prevent powdery mildew, but they will not cure or remove existing infections; they can only prevent further spread. Management with fungicides can be difficult and may require repeated treatments.  Some of the articles linked below have more information on the use of fungicides.

Want a little more information? Here are some links:

Clemson University Extension Powdery Mildew Fact Sheet

Purdue University Extension:  Plant Disease – Powdery Mildew

University of Minnesota: Powdery mildew in the flower garden

Purdue University Extension: Disease Resistant Annuals and Perennials (although this is from 2009 and there are many more disease resistant choices now)

Cornell: Disease Resistant Vegetable Varieties